The field of magnetic resonance is well established in research and commercial applications. For example, nuclear magnetic resonance (NMR) can measure the transitions between quantum-mechanical spin states of atomic nuclei, such as 1H, 13C, and 15N. In another example, electron spin resonance (ESR) can measure transitions between quantum-mechanical spin states of electrons. In both cases, the simplest type of measurement is linear spectroscopy, which involves measurement of a signal that is linearly proportional to the amplitude of incident electromagnetic radiation. This signal can be used to estimate the spin transition frequency. In some cases, a static or “DC” magnetic field, labeled B0, can be applied, and the transition frequency is typically proportional to the magnitude of B0. In addition, new transitions can be generated and the original transition peaks are split and shifted due to Zeeman interactions. The transition frequency in all cases are dependent on the magnitude of B0.
Existing techniques typically measure NMR transitions having frequencies in the range of about 50 MHz to about 1000 MHz (in the radiofrequency, or RF, range). For ESR transitions, only those in the frequency range of about 5 GHz to less than 1000 GHz (in the microwave range of the spectrum) are explored.
There are also various ESR transitions in the THz regime. For example, in small molecules or large molecules including biomolecules such as proteins, some ESR transitions may be highly sensitive to molecular structure and environment including the identities and geometries of ligands of the ions in question. Thus the transitions can be used for identification of chemical species or for characterization of the ligand binding or molecular configuration around ions. They can also be used to distinguish different possible molecular configurations including those of biomolecules such as proteins. However, due to the limitations of available THz light sources, these THz ESR transitions remain largely unexplored.